German Patent Publication DE 29 00 295 C2 discloses an angular measuring apparatus wherein the transparent areas of the index disk which are remote from the illumination source are larger than the transparent areas on the index disk which are closer to the illumination source. The objective of such an arrangement is to allow the largest amount of light energy possible to strike the photo detectors.
It has been shown that with systems of this type, the amount of light received by the detectors depends upon the distance of the individual index disks from one another and the distance of the individual index disks from the illumination source. Furthermore, the phase relationships of the detected signals are undefined with respect to illumination sources which emit diverging or converging beams of light. In addition, a reduction in the scanning distance between the illumination source and index disk results in an increase in the light energy detected and thus to an increase in signal amplitudes at the output of the detectors. This may result in saturation of the downstream electronic evaluation unit and thus should be avoided.